At present, a conventional projection lens usually has several deficiencies such as a large focal length, a small field of view, and a large throw ratio. However, with the development of projectors, projection and display of large size pictures become increasingly popular among consumers. A common projection lens can no longer satisfy a requirement of projection of a large-size picture in a small space, and such a requirement becomes one of the important reasons that restrict the rapid development of projectors. Although some existing projectors have a throw ratio less than 1, for reasons such as low contrast, relatively poor resolution, large chromatic aberration, and relatively large distortion, such projectors cannot fulfill market demands. With the ongoing development of projection technologies, a throw ratio of an ultra-short distance projection lens may already be less than 0.3, which well satisfies the requirement of projection of a large-size picture in a small space.
An existing ultra-short distance projection lens has a complex structure. Generally, multiple aspheric lenses are applied to correct aberrations. However, the multiple aspheric lenses may lead to a complex structure of the ultra-short distance projection lens, undesirable tolerance, and a high demand on the precision of assembly. Meanwhile, in some ultra-short distance projection lenses, a free-form reflective mirror is used to effectively correct distortion and eliminate astigmatism. Compared with a rotationally symmetric reflective mirror, a free-from reflective mirror can keep distortion and astigmatism at a low level. However, it is very difficult to precisely test a free-form reflective mirror, and therefore it is no easy to control the shape of the free-form surface in mass production. Meanwhile, the free-form reflective mirror has a very strict requirement on tolerance and a high demand on the precision of assembly, leading to a low yield in the mass production. Moreover, in some ultra-short distance projection lenses, a rotationally symmetric reflective mirror is used. Regardless of advantages such as simple structure, easy testing, and easy production, a rotationally symmetric reflective mirror has different curvature distribution in the meridional direction and in the sagittal direction, thus causing a large astigmatism. The overlapping meridional intermediate image and sagittal intermediate image are separated by the rotationally symmetric reflector, which means that the projection lens cannot throw a clear image.